At present it is customary for aircraft to furnish the power required by electric onboard systems with the aid of an auxiliary power unit (APU) while on the ground and with the aid of generators in the engines of the aircraft in flight operation.
Owing to local noise and exhaust gas regulations, the use of the APU on the ground is partly restricted. In such cases there is a possibility of meeting the electric power requirements through a ground power unit (GPU) or air starter unit (ASU) that is present externally of the aircraft. A GPU or an ASU is moreover employed when the battery of an APU of the aircraft is exhausted so that it is not possible to start the APU of the aircraft. This may bring about delays in air traffic. As the provision of a GPU or ASU is a service on the part of the respective airport, making use of this incurs additional costs for the operator of the aircraft.
In the case of aircraft that are to meet a particular requirement profile of the ETOPS (Extended-range Twin-engine Operation Performance Standard) it is mandatory for certain flight routes to add on the APU in flight operation so as to ensure a sufficient supply of electric current for the case that an engine and thus the generators integrated therein should fail. As a result the fuel consumption increases, the maintenance interval for the APU is shortened, and the operating costs of the aircraft are increased accordingly.
Furthermore it may happen that an aircraft having left the parking position still has to wait on a taxiway of an airport for several hours because the runway is closed, e.g. due to an incident. Particularly in the case of long-haul aircraft it is then frequently necessary to request a GPU as well as an air conditioning truck, for without the power supply from the generators driven by the running engines it is not possible to guarantee sufficient air conditioning of the aircraft's cabin.
I.e., the energy consumption of an aircraft occupied to capacity can not be covered in every situation without engines running, particularly when on the ground.